A. Technical Field
The present invention relates to a micromechanical sensor comprising a substrate and at least one mass which is situated on the substrate and which moves relative to the substrate for detecting motions of the sensor based on an acceleration force and/or Coriolis force which occur(s), the mass and the substrate and/or two masses which move toward one another being connected by at least one bending spring device for a relative rotational motion.
B. Background of the Invention
Micromechanical sensors are used for detecting accelerations and/or yaw rates along a spatial axis or at least one of three mutually orthogonal spatial axes. The operating principle is basically that a sensor mass is moved relative to a substrate as a response to the corresponding acceleration or yaw rate of the sensor. For this purpose, the sensor mass is movably mounted on the substrate by means of a bending spring device, which is generally composed of one or more bending springs. The design of these bending springs primarily determines the particular directions in which the sensor mass is movable. The spring stiffnesses of the bending springs are different in the individual spatial directions in order to more or less permit different bending directions. This difference in movability may be influenced by varying the cross-sectional surface area of the bending spring, and also by virtue of the spatial course of the bending spring. In particular for a meandering design of the bending spring, relatively high elasticity may be achieved in the plane of the meander. Ideally, the bending springs are linear, and in this case are characterized by a single spring constant. “Linearity” means that a constant force acting in the direction of the provided deflection direction always results in the same deflection of the spring, regardless of how strongly the spring has already been deflected. However, this is not the case for the bending springs in sensors of the prior art. Evaluation of the acting forces is therefore difficult and prone to errors.
Micromechanical sensors are known from U.S. Pat. No. 6,705,164 B2 and WO 01/20259 A1 in which sensor masses are attached to other movable masses by means of a bending spring device, or to a substrate by means of an anchoring. The bending spring device is composed of multiple individual bending springs which have meanders. Elastic movability between the two masses or between the mass and the substrate or anchoring is achieved as a result of the meanders. The meanders are each attached to the components of the sensor, which are movable relative to one another, via short stubs. The movability of the bending spring device essentially results from the meanders themselves, not from the stubs. A disadvantage of these bending spring devices of the prior art is that, for the same force, the bend of the bending spring device cannot be linearly deflected. This is particularly disadvantageous since, for example, for the same Coriolis force the path of deflection of the corresponding sensor mass becomes smaller with increasing deflection. This results in errors in detecting the associated acceleration or yaw rate values, or, in order to compensate for the errors, requires complicated computations which likewise adversely affect the accuracy and the speed of display of the acceleration or yaw rate.
A bending spring device is known from U.S. Pat. No. 6,954,301 B2 which extends in a meandering shape on both sides of short fastening stubs. Although linearity of the bending spring device may be improved in this manner, a controllable stiffness of the bending spring device is difficult to achieve. In addition, the spring is asymmetrical due to the S-shaped curve, which may result in parasitic effects which are difficult to control. Such a bending spring device is generally very soft, and it is difficult to introduce a predefined stiffness into this bending spring device.
The object of the present invention, therefore, is to provide a micromechanical sensor in which a bending spring device is present that allows a deflection of the bending spring device which is as linear as possible.
The object is achieved by a micromechanical sensor having the features of claim 1.
A micromechanical sensor according to the invention has a substrate and at least one mass which is situated on the substrate and which moves relative to the substrate for detecting linear and/or angular accelerations of the sensor. On the one hand, the mass moves in the sense of a drive motion form, which in the absence of external accelerations is stationary, and on the other hand responds with detection motions when acceleration forces and/or Coriolis forces act on the sensor. The moving sensor mass is attached to the substrate by means of at least one bending spring device. Alternatively, multiple masses which move toward one another may be connected by at least one bending spring and moved relative to one another. Consequently, it is not necessary in each case for the sensor mass to be situated directly on the substrate. In some embodiments of micromechanical sensors according to the invention, the sensor mass may also be attached to a drive mass, for example, and together with the drive mass moved as a primary motion, and moved relative to the drive mass only for indicating an acceleration force and/or Coriolis force. The sensor mass and the drive mass are then connected to one another via the corresponding bending spring device. The bending spring device is designed in the form of at least one meander.
According to the invention, the bending spring device has multiple, in particular two, spring bars extending essentially parallel to one another for improving the linear characteristic, i.e., the linear spring characteristic, of the bending spring device during the rotational motion, and at least one meander on at least one, preferably on all, of the spring bars.
The design provided according to the invention for the bending spring device by means of a multiple spring bar having at least two spring bars extending essentially in parallel allows a targeted, predeterminable flexural strength of the bending spring device. The additional provision of meanders on at least one, preferably on all, of the spring bars contributes very advantageously to the linearity of the bending characteristic of the bending spring device. The bending spring device according to the invention is therefore linearly and uniformly bendable, at least with regard to deflections in the plane in which the meanders are situated. According to the invention, not only double spring bars, but also multiple spring bars having more than two spring bars are possible, although the double spring bars described below are particularly advantageous. In addition, multiple meanders are possible. By combining the multiple spring bars, which take part in the bending of the bending spring device, with meanders associated therewith, a spring characteristic is obtained such that the linearity of the bending spring device is provided over the entire range of its deflections.
It is particularly advantageous when at least two of the spring bars, but preferably all of the provided spring bars, extend essentially parallel to one another. A particularly uniform provided stiffness of the bending spring device is thus achieved, the bending spring device being substantially linearly deflectable over a large bending range. In another embodiment of the invention, however, it may also be provided that individual spring bars or all of the spring bars extend not in parallel, but tapering toward one another, for example. A curved design of the spring bars may also be advantageous, in which case the parallelism of the spring bars no longer has to be provided. The most favorable embodiment of the invention should be selected in each case, depending on the application and the required stiffness of the bending spring device.
If the meander is designed in such a way that it merges into the spring bar in a rounded manner, stresses which are caused by bending may be achieved which are more uniform and which do not have unacceptable peaks, even in extreme bending situations.
Similarly as for the meander merging into the spring bar in a rounded manner in order to avoid stress peaks, it is advantageous when the spring bar(s) likewise merge(s) in a rounded manner into the adjacent component, in particular the sensor mass or the substrate or an anchoring for attachment to the substrate. Stress peaks are thus reduced not only in the region of the meander, but also in the remainder of the bending spring device. Of course, the rounded transition may also be provided without the rounding at the meander.
In one advantageous embodiment of the invention, another measure for reducing and evening out the stresses in the bending spring device under load may be achieved by the rounded transition having a nonuniform progression, i.e., a non-constant radius of curvature. The meanders as well as the spring bars are thus connected to the adjacent components in a particularly gentle manner with regard to their stresses. The uniformity of the relation between the applied force or applied torque and the resulting deflection, and the associated accuracy of the measurement by the sensor, are thus improved.
It is particularly advantageous when the rounded transition is elliptical. As a result, it is possible in particular to reduce peak stresses in the springs, which may be introduced into the sensor due to an externally acting shock event, and thus make damage to the sensor more unlikely.
In one particularly advantageous embodiment of the invention, it is provided that the meander and/or the spring bar merge(s) in a branched manner into the sensor mass, the substrate, and/or an anchoring for attachment to the substrate. Stress peaks in the transition points are thus additionally reduced. The linear deflection of the bending spring device is assisted in this manner.
When the meander and/or the spring bar has/have an elliptical bend or a convex or concave curvature, this supports a bending characteristic which reduces stress peaks even in extreme situations, such as mechanical shock events, for example. Damage to the sensor is thus largely avoided. The linear bending characteristic of the bending spring device improves the signal evaluation by the sensor.
The meander is advantageously situated off-center with respect to the spring bar, in particular, closer to moving components than to the stationary anchoring of the bending spring device. This results in different lengths of the spring bar on each respective side of the meander. The length of the particular spring bar section should be selected depending on the application and the desired flexural strength. However, it has been shown that, in particular for a stationary part and a moving part, or when a less intensely moved component is connected to a more intensely moved component, the meander should be situated closer to the more intensely moved component. The spring bar section present between the meander and the more intensely moved component should therefore be smaller than the section facing the less intensely moved component or the stationary component.
The distance between the bars of the meander is preferably less than the distance between the two spring bars. It has been shown that particularly high linearity of the bending spring device is achievable using such a design. In any case, it must be ensured that contact between the individual spring bars is avoided during severe bending of the bending spring device. Such contact would result in damage and intense interference with the linearity requirements of the bending spring device.
When the distance between the two spring bars is a multiple of the width of the spring bars, a bending spring device is advantageously provided which is very elastic and has individual bending bars, but still has relatively high overall strength. In addition, contact of the individual spring bars is avoided due to the large distance.
The length of the two spring bars is advantageously greater than the length of the bars of the meander. As a result, the important bending motion of the bending spring device is provided by the spring bars themselves. Only one compensating structure is provided by the meander in order to support the linearity of the bending spring device.
When the spring bars of the double spring bar and/or the bars of the meander have a symmetrical design, uniform bending of the bending spring device in both provided directions is ensured. On the other hand, one-sided bending may be provided by an asymmetrical design of the bending spring device.
To allow particularly harmonic and uniform bending without stress peaks on individual parts of the bending spring device, it is advantageous when the meander has a radius of curvature having an internal midpoint. The radius of curvature should be as large as possible in order to obtain a small curvature of the meander. Stress peaks in the region of the turns or inflections of the spring are thus reduced.
When the meander has at least one further radius of curvature having a midpoint outside the meander, so that the meander has a convex curvature, a particularly harmonic and uniform transition is provided which likewise contributes to the reduction of stress peaks.